Impact of salinity on element incorporation in two benthic foraminiferal species with contrasting magnesium contents

Geerken, Esmee; de Nooijer, Lennart Jan; van Dijk, Inge; Reichart, Gert-Jan

Accurate reconstructions of seawater salinity could provide valuable constraints for studying past ocean circulation, the hydrological cycle and sea level change. Controlled growth experiments and field studies have shown the potential of foraminiferal Na inline-formula∕ Ca as a direct salinity proxy. Incorporation of minor and trace elements in foraminiferal shell carbonate varies, however, greatly between species and hence extrapolating calibrations to other species needs validation by additional (culturing) studies. Salinity is also known to impact other foraminiferal carbonate-based proxies, such as Mg inline-formula∕ Ca for temperature and Sr inline-formula∕ Ca for sea water carbonate chemistry. Better constraints on the role of salinity on these proxies will therefore improve their reliability. Using a controlled growth experiment spanning a salinity range of 20 units and analysis of element composition on single chambers using laser ablation-Q-ICP-MS, we show here that Na inline-formula∕ Ca correlates positively with salinity in two benthic foraminiferal species (Ammonia tepida and Amphistegina lessonii). The Na inline-formula∕ Ca values differ between the two species, with an approximately 2-fold higher Na inline-formula∕ Ca in A. lessonii than in A. tepida, coinciding with an offset in their Mg content (inline-formula∼ 35 mmol molinline-formula−2 versus inline-formula∼ 2.5 mmol molinline-formula−1 for A. lessonii and A. tepida, respectively). Despite the offset in average Na inline-formula∕ Ca values, the slopes of the Na inline-formula∕ Ca–salinity regressions are similar between these two species (0.077 versus 0.064 mmol molinline-formula−1 change per salinity unit). In addition, Mg inline-formula∕ Ca and Sr inline-formula∕ Ca are positively correlated with salinity in cultured A. tepida but show no correlation with salinity for A. lessonii. Electron microprobe mapping of incorporated Na and Mg of the cultured specimens shows that within chamber walls of A. lessonii, Na inline-formula∕ Ca and Mg inline-formula∕ Ca occur in elevated bands in close proximity to the primary organic lining. Between species, Mg banding is relatively similar, even though Mg content is 10 times lower and that variation within the chamber wall is much less pronounced in A. tepida. In addition, Na banding is much less prominent in this species than it is in A. lessonii. Inter-species differences in element banding reported here are hypothesized to be caused by differences in biomineralization controls responsible for element uptake.

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Geerken, Esmee / de Nooijer, Lennart Jan / van Dijk, Inge / et al: Impact of salinity on element incorporation in two benthic foraminiferal species with contrasting magnesium contents. 2018. Copernicus Publications.

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Rechteinhaber: Esmee Geerken et al.

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